Astronomers have made a groundbreaking discovery by identifying ancient stars that originated during the Cosmic Dawn in the central region of the Milky Way galaxy. The search for these elusive stars, as part of an extensive survey to uncover the oldest stars known to humanity, has shed light on their relatively calm rotation around the galactic center, despite the chaotic environment surrounding them. Astronomer Anke Arentsen of the University of Cambridge in the UK led the research and presented the findings at the annual National Astronomy Meeting in the UK.
The age of a star can be determined by its metal content. When the first stars in the Universe emerged, they had to form from the elemental materials available at that time, primarily hydrogen and helium. However, the nuclear reactions occurring in their cores initiated the fusion of hydrogen atoms into heavier elements, ranging from helium to iron. When these stars exploded in supernovae, they dispersed these heavier elements throughout space, along with even heavier elements produced in rapid and energetic supernova processes. Consequently, subsequent stars formed with a higher proportion of heavy elements. Thus, the younger a star is, the greater its metal content is likely to be. Conversely, older stars have lower metal content. Pristine stars have been discovered throughout the Milky Way, primarily in its outskirts and the galactic halo. However, astronomers believe that the oldest stars should predominantly reside in the galactic center. The challenge lies in their detection due to the abundance of metal and the obstructive presence of dust, hindering our view.
To overcome these challenges, Arentsen and her colleagues initiated the Pristine Inner Galaxy Survey (PIGS). By analyzing the spectrum of light emitted by a specific star, astronomers can identify wavelengths that are either amplified or dampened by the presence of particular elements. They searched for an elemental signature consistent with stars that have very low metal content and successfully identified approximately 8,000 potential candidates. Through subsequent observations, the researchers confirmed the chemical compositions of these metal-poor stars, resulting in a selection of around 1,300 ancient stars located in the galactic center. The abundance of discoveries enabled the team to conduct population studies.
The researchers utilized data obtained from the Gaia observatory, an ongoing project dedicated to mapping the three-dimensional positions and movements of stars within the Milky Way. This data enabled them to determine the galactic orbits of the ancient stars. Arentsen and her colleagues discovered that these stars have relatively slow orbits around the galactic center. Furthermore, the older stars exhibit more erratic orbits, yet they maintain an average orbit around the center. Interestingly, the orbits of these stars are primarily confined within the galactic center. Even stars with elliptical orbits tend to remain within the central region of the Milky Way.
Arentsen expresses her excitement about the significance of these findings, stating, “It is exciting to think that we are seeing stars that formed in the earliest phases of the Milky Way, previously largely out of reach. These stars likely formed less than a billion years after the Big Bang, so are relics from the early Universe.” She also anticipates further discoveries, remarking, “The available data for these ancient objects is growing rapidly. I’m excited to see what we will learn about these first stars to populate our Galaxy in the next few years!”
The discovery of ancient stars in the heart of the Milky Way provides valuable insights into the early phases of star formation in our galaxy. The identification of these stars, despite the challenges posed by their low metal content and obstructed visibility, offers a glimpse into the origins of the Universe. With ongoing advancements in data collection and analysis, the scientific community eagerly awaits the revelations that await us regarding these ancient celestial entities.
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